The present invention relates generally to ceramic piston structures, and more particularly to a ceramic piston head which is secured to the top portion of a piston in a manner which prevents cracking the ceramic piston head and its separation from the piston.
Recently, a piston has been developed which improves the combustion efficiency of diesel engines or the like, and has a piston head made of a ceramic material. The ceramic piston head prevents the scattering of heat from the combustion chamber, and thereby suppresses the production of unburnt gas.
Various structures have been proposed for firmly securing a ceramic piston head to the top portion of a piston. A typical example of such known structures employs an adhesive by which the piston head is adhered to the piston body. This structure, however, is impractical because the adhesive is degraded by the heat generated by the engine and allows, in the worst case, the separation of the ceramic piston head from the piston body.
Another known structure is provided with screw threads formed in the ceramic piston head and the piston body so that these two members are firmly secured to each other by a screwing engagement. This structure is also impractical because it necessitates a complicated and troublesome machining for forming the screw threads and may allow the piston and ceramic head to loosen from each other at the screwing portion of the piston assembly.
Still another more practical structure employs a shrink fit between the ceramic piston head and the piston body. This known structure, however, may be subject to separation and dropping of the piston body from the ceramic piston head when the engine is operated at a high temperature. Such separation is the result of a large difference between the respective coefficients of thermal expansion of the metal constituting the piston body and the ceramic of the piston head. However, if the shrink fit is made by producing a large shrinkage so as to prevent the dropping of the piston head, the ceramic may crack as a result of an excessive tightening force.
It has been further proposed that the piston head might be secured to the piston during casting of the piston body. This proposal, however, has the disadvantage that the ceramic is likely to be broken when the metal of the piston body solidifies. To avoid this, it has been attempted to place a glass fiber, or the like, as a buffer between the ceramic and the metal. However, such a structure tends to loosen the fit between the ceramic piston head and the piston body because the glass fiber tends to be crushed by the engine vibration during operation.
To overcome this problem, a proposal has been made to use a metallic ring 3 interposed between the metallic piston body 2 and the ceramic piston head 1, as shown in the prior art arrangement of FIG. 1. In this manner, the tightening force which is increased as the molten metal of the piston body solidifies is absorbed and relaxed by the metallic ring 3.
This known structure is effective in preventing the cracking of the ceramic piston head. However, the metallic ring 3 cannot provide a secure fit intermediate of the piston body 2 and the ceramic piston head 1 because the known metallic ring 3 has smooth inner and outer peripheral surfaces. Therefore, a slight gap is formed between the metallic ring 3 and the metallic piston body 2 or between the metallic ring 3 and the ceramic piston head 1 due to the difference between the coefficients of thermal expansion of the metal of the ring 3 and the metal of the piston body 2, or due to deterioration of stiffness of the metal, so that the piston head 1 may be dangerously separated from the piston body after an extended period of operation.
For these reasons, the prior art has not produced a practicable piston having a ceramic piston head.
It is, therefore, an object of the invention to enhance and improve the fit of a metallic ring between a ceramic piston head and a metallic piston body so as to provide a reliable piston structure which avoids the problems of loosening and rattling, even after a long use, and thereby overcomes the prior art problem of dropping of the ceramic piston head.